Author Topic: Cloud movement and direction  (Read 18330 times)

Lost Soul

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Re: Cloud movement and direction
« Reply #15 on: August 08, 2012, 07:28:51 PM »
Adi,

To respond to your post.

From what I read of your post you are saying that the surface wind blows at 0° degrees then it can only blow +/-30° above the surface at 2000'.

More or less correct.   To be specific +30° in the Northern Hemisphere and -30° in the Southern Hemisphere

So are you saying the wind can only change direction by 30° in a 2000' layer of space?  You have lost me completely.

Yes , let me help your understanding.  A fundamental and undeniable fact and proven principal of atmospheric physics is that the atmosphere consists of high and low pressure systems constantly on the move.  Each system covers many hundreds of square miles of the Earth's surface.  These pressure systems have pressure gradients across them.  These gradients are represented by isobars on a weather chart in exactly the same way that we represent gradients (hills and valleys) across the landscape by contour lines.  At heights at above 2000 feet and above the air moves around the pressure systems parallel to the isobars. 

This movement of the air is of course the wind.  At heights above 2000 feet other factors may or may not come into play and disturb the direction of that flow,  Below 2000 feet friction generated by the Earth's surface causes this air movement to slowdown and in so doing change direction by 30°.  So if you are in the northern hemisphere and the wind is blowing 0° at 2000 feet on the ground level it will be 330°.  i.e backed by 30°.

This fundamental principle is taught on day one on any metrological course.  Certainly to forecasters, met observers and anyone else who has an interest in the upper winds.  It is one of the cornerstones of meteorology.  If you want a good brief explanation of it then I recommend you to read the Air Pilots Weather Guide by Ingrid Holford.  Chapter 3 deals with pressure winds.

Not many aircraft fly at 2000'.

Yes they do.  Most light aircraft to fly at that height.  When pilots are being taught to fly this is the altitude of choice for various reasons.  Not least of which is that we tend to suffer low cloud bases in this country and the vast majority of light aircraft must maintain visual contact with the ground below.  Because they just do not have the instrumentation required for them to fly in cloud and neither do the pilots have the appropriate training or qualification to do so. 

So as you can well imagine there are many hundreds of flights per day at 2000 foot level or there abouts.  Also many helicopters fly at these levels or lower.  The vast North Sea offshore helicopter support operations running out of Aberdeen and Sumburgh fly at the 2000 to 3000 foot level.  And on a good day there are more flights across the North Sea Oil fields by helicopters then there are out of Heathrow on a peak summer weekend.

If you ever get the chance to storm chase a supercell storm you can see vertical shear of greater than 30° in action.

Yes I am fully aware of the vertical shear that exists in a super cells or even lesser thunderstorms.  The vertical winds traveling both up and down to devastating effect.  On the down motion it can burst out of the bottom of the storm cloud hit the ground with considerable energy and disperse in all directions of the compass.  Its effect being felt up to 20 miles away.  This is what is known as a micro burst and is of course a localised atmospheric wind effect.

I have just found this video that clearly shows the low level cloud travelling in the opposite direction from the upper level cloud.

Yes the video demonstrate horizontal wind shear.  Probably a very localised phenomena brought about by features of the landscape.  If you know where to look on the Internet you can find all sorts of videos showing weird and wonderful weather effects.  But they do not disprove the fundamental principles of air mass movement around an atmospheric pressure system.  Surface winds are affected by all sorts of low-level effects such as hills, valleys, buildings and surface heating. 

Which is something you are clearly very conversant with and is also discussed in the book reference given above.  As I said in my very first post wind direction is notoriously fickle and difficult to predict; particularly at ground level for the reasons given above.  However, on open ground such as the Essex Marshes, Exmoor the tops of the South Downs etc those effects don’t really exist other than perhaps on-shore breezes on a very hot day, so the wind backing by 30° at the surface will hold good.  But there again for an on shore breeze to form to any great effect general surface winds have to be light.  Which indicates the presnce of a high pressure system, clear skies and no likely hood of any weather.

And yes I appreciate that for your local area you like to predict convective weather (ultimately thunderstorms I assume ) from tepigrams.   Building up a data base, if only a mental one, of what sort of patterns in a tepigram will give a probability of a thunderstorm in your local area is a neat skill indeed.  However, what we have done I believe is both answered Hugh Westacott’s question from differing view points.

As he says he just wants to be able to tell his ladies if they need to keep their water proofs on or not.  And I very much doubt if they will be venturing out when the weather is serious and thundery or in remote mountain areas.  Interestingly MLTs Hill Walking Manual which discusses weather doesn’t go into any of this stuff about upper winds that you and I have been going on about. Unfortunately, we have done what so often happens in these forums.  We have hijacked a simple question and set it off spinning in a different irrelevant direction.  All Hugh required was a simple, practical, instantly usable answer to a simple question.

As Skills4Survival says in his posts: Yes, the prevailing wind I am clear on. Also, out of which direction the wind comes...I can see with my compass, I just do not understand the whole difficulty of determining the cloud thing

1. step one, feel wind
2. step two read compass
3. do not forget..and use it if needed.

So wind, understand, cloud direction...that part I do not see yet, why that would really add value to what we would want to achieve.
 
I just use the wet finger a few times before I pitch camp.


That neatly brings it all back on track and we should leave it there.

Skills4Survival

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Re: Cloud movement and direction
« Reply #16 on: August 08, 2012, 10:02:23 PM »
Just to make sure, it was not meant in a bad way. I understand a little bit of what has been said, just because I studied geography a long time ago. I honestly did not see the fit into practical navigation or survival for that matter (but open for suggestions!)

It is however insightfull and I thank you for it. I do believe that knowing a little bit about wind, weather systems, high/low pressure, pressure changes, etc. can be helpful in specific situations.

greetz,

Ivo

Ivo

Rescuerkw

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Re: Cloud movement and direction
« Reply #17 on: August 08, 2012, 11:43:27 PM »
Well guys you've completely lost me with all that technical stuff. In the lowland areas of East Anglia where I reside, the prevailing wind in a useful indicator of direction, but of course we don't have those large protrusions in our landscape that you guys call hills and mountains. In our 'big sky' part of the UK an approaching weather change is usually quite easy to see and because it's not subject the massive terrain undulations that one will find in the Peaks or the Lake district, it tends to be a lot more predictable and more often than not does what it looks like it going to do. So, when I'm out with the ladies and I see a big black cloud approaching, I usually suggest that we all put our waterproofs on. As you can tell, an incredibly scientific approach - but in fact as you will know if you've read some of my other posts I love to keep all this stuff as simple as possible (maybe a reflection of my character?)
« Last Edit: August 08, 2012, 11:46:39 PM by Rescuerkw »

Skills4Survival

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Re: Cloud movement and direction
« Reply #18 on: August 09, 2012, 09:07:15 AM »
I do the same and sometimes I do it in the morning  :D . If possible I know the general (local) wheather forecast, more related to day planning if needed and just be a bit more comfortable on possible risks, e.g. snowfall and prepare smartly.
Ivo

Lyle Brotherton

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Re: Cloud movement and direction
« Reply #19 on: August 09, 2012, 11:48:10 AM »
I have learned more from this thread than all the books I mentioned earlier, really interesting and some fantastic info.

You guys are experts - Ron, I am currently at your level of understanding ;)
« Last Edit: August 09, 2012, 08:59:37 PM by Lyle Brotherton »
“Opinion is the medium between knowledge and ignorance” - Plato

adi

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Re: Cloud movement and direction
« Reply #20 on: August 09, 2012, 09:38:13 PM »
I will point out you are correct about the surface wind being at 30° to the Isobars of a pressure system this is known as the angle of indraft.

I have just emptied my shelf of all my meteorological books and I am now sat in the middle of them.
« Last Edit: August 09, 2012, 10:29:14 PM by adi »
"We do not belong to those who only get their thought from books, or at the prompting of books - it is our custom to think in the open air, walking, leaping, climbing or dancing, of lonesome mountains by preference, or close to the sea, where even the paths become thoughtful." Friedrich Nietzsche

adi

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Re: Cloud movement and direction
« Reply #21 on: August 10, 2012, 12:02:43 AM »
Right I have got to the bottom of this. Quoting the book Weather and Forecasting by Storm Dunlop & Francis Wilson, Octopus Publishing Group, 1982 'The angle which the winds make to the Isobars depends on the surface friction and will be at least (about 10° - 20° over sea and greater (about 25° - 35°) over land. This frictional effect lessons with height and so gives rise to a shift in the wind direction. At 500 - 1000 meters (1640 - 3280 ft) the wind is flowing freely along the isobar.' Cloud is usually above this dirty air so are not effected by friction so normally show the real wind direction although it is important to remember wind at different heights can go in different directions.   

One small paragraph in one book out of 12!   

LOL I wish i did only predict convective weather for my location, I produce convective forecasts for the States, much of Europe and the whole of the UK depending where people are at the time. Today I gave two general 3 day forecasts, one was for Winston-Salem, North Carolina and Washington, Virginia and a convective forecast for Whitesburg, Kentucky, this one is for a local radio station that I have a contract to give out of hours weather updates when they are under a watch.
« Last Edit: August 10, 2012, 01:07:48 AM by adi »
"We do not belong to those who only get their thought from books, or at the prompting of books - it is our custom to think in the open air, walking, leaping, climbing or dancing, of lonesome mountains by preference, or close to the sea, where even the paths become thoughtful." Friedrich Nietzsche

Callum

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Re: Cloud movement and direction
« Reply #22 on: August 10, 2012, 08:23:37 AM »
This posting has taken a lot of re-reading for me to get to grips with, and I think that I have now.

Lost Soul and Adi, yes it has spun off in different directions, but I personally am pleased that it has, as it has made me study the subject in greater detail and actually fired up my interest, so again, thanks guys :)

Pete McK

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Re: Cloud movement and direction
« Reply #23 on: August 10, 2012, 05:41:16 PM »
Like Cal, I have really enjoyed this thread, which book would you guys recommend as a beginners introduction?

Skills4Survival

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Re: Cloud movement and direction
« Reply #24 on: August 10, 2012, 06:00:52 PM »
you could look at chapter 8 of "hillwalking" of steve long. I also like "how to identify the weather" by storm dunlop 2002, harper collins
Ivo

adi

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Re: Cloud movement and direction
« Reply #25 on: August 10, 2012, 06:00:59 PM »
wow Pete that is a hard one, it depends how technical you want.

This might be a good place to start it covers the basics
http://www.amazon.co.uk/Weather-Hillwalkers-Climbers-Leisure-Interests/dp/0750910801

"We do not belong to those who only get their thought from books, or at the prompting of books - it is our custom to think in the open air, walking, leaping, climbing or dancing, of lonesome mountains by preference, or close to the sea, where even the paths become thoughtful." Friedrich Nietzsche

Lost Soul

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Re: Cloud movement and direction
« Reply #26 on: August 10, 2012, 06:50:55 PM »
From a practical point of view to get you started with very little technicalities involved.  I suggest you try Chapter 8 of Hill Walking by Steve Long.  That will give you a basic introduction.  Follow that up with Chapter 3 of Winter Skills by Cunningham and Fyfe.  That gives a top for winter conditions.  Both published by Mountain Leader Training.

I am not sure if this one is still published but it’s very useful.  Instant Weather Forecasting by Alan Watts published by Adlard Coles.  Basically a pictorial guide to sky conditions and what they mean and what you can expect in terms of approaching weather. Actually that would be useful to Hugh re his original posts – when do I advise my ladies that they can remove thier waterproofs / they need to keep them on.

On a similar vein try the Cloud Book by Richard Hamblyn, Published by the Met Office.

And Adi, re your post of this morning I am pleased we concur on all points raised.
« Last Edit: August 10, 2012, 07:02:03 PM by Lost Soul »

adi

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Re: Cloud movement and direction
« Reply #27 on: August 10, 2012, 07:05:22 PM »
Yeah Instant Weather Forecasting by Alan Watts is a good book to learn now-casting by looking at the sky.

And one of the best books on synoptic weather is 'Outlook Weather maps and Elementary Forecasting' By G.W. White, first published in 1967. unfortunately it is like rocking horse poo to get now. And some of the stuff covered has been superseded by better understanding and modern technology but it is one of the best books for describing air masses and frontal systems.
"We do not belong to those who only get their thought from books, or at the prompting of books - it is our custom to think in the open air, walking, leaping, climbing or dancing, of lonesome mountains by preference, or close to the sea, where even the paths become thoughtful." Friedrich Nietzsche

Hugh Westacott

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Re: Cloud movement and direction
« Reply #28 on: August 19, 2012, 06:51:49 AM »
Thanks chaps for all your responses.

The reason for posting my question bordered on the frivolous but I feel vindicated because it resulted in a fascinating discussion from which I, and I suspect other members of this forum, have learnt a lot.

Hugh

adi

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Re: Cloud movement and direction
« Reply #29 on: August 19, 2012, 05:51:57 PM »
Not a problem Hugh there is no such thing as stupid question only stupid answers!

I leaned something from lost Soul. surface wind measurements for the meteorologist are measured at 10 meter above the ground and these blow at 30° to the isobars at this height thanks to friction. What I learnt was that from 500 to 1000 meters the wind is no longer effected by friction. Not that important for server weather forecasting but hugely important for aviation. 
"We do not belong to those who only get their thought from books, or at the prompting of books - it is our custom to think in the open air, walking, leaping, climbing or dancing, of lonesome mountains by preference, or close to the sea, where even the paths become thoughtful." Friedrich Nietzsche